Notes and Observations on a Guan. 293 



Committee of Inquiry, 1911), to the fact of the special selec- 

 tive function of the csecal colon in this bird, whereby the 

 rough fibres and undigested debris of heather and other 

 vegetable matter, present in the alkaline chyme when it 

 reaches the specialized colic caecum, are passed on through 

 the sphincteric tract to the colon ; while the more fluid 

 contents are pressed back into the caeca proper, where they 

 undergo an acid digestion through, probably, the agency of 

 bacteria. 



Garrod, again, referring to the osteology of the Hoatzin 

 (P. Z. S. 1897, p. 109), says : '' I may, however, mention that 

 it is only in the Cracidse, among allied birds, that the vomer 

 runs so far forward in the palate, at the same time that it is 

 tumified at its anterior extremity. In Ortalis albiventris this 

 is most strikingly the case." 



Again, he goes on to say : '^ The presence of two carotid 

 arteries, an ambiens muscle, an accessory femoro-caudal, and 

 a deep plantar vinculum place its non-passerine nature 

 beyond a doubt. Adding the tufted oil-gland and the inch- 

 long colic caeca, the bird could only be related to the Tina- 

 midae, Gallinae, or Rallidje/^ 



That it can have nothing to do with the Rallidae is evident 

 from the fact that Opisthocomus is holorhinal, and Garrod 

 concludes that it must therefore be a Gallinaceous bird or 

 form a group by itself. He adds, that " as there is no 

 Gallinaceous bird without a direct articulation between the 

 pterygoid bones and the basi-sphenoidal rostrum, it is 

 hardly possible to include the Hoatzin along with them ; and 

 yet it resembles them most clearly, as it does the Cuculidae, 

 in the length of its colic casca and the number of its 

 rectrices." Indeed, as he suggests, it would appear very 

 probable that Opisthocomus left the parent stem very shortly 

 before the true Gallinae first appeared. 



Description of a young bird. 



This specimen, which is labelled Ortalis vetula (Wagl.), 

 Mexico, Cuesta de Misantla, Junio 1888, No. 91, 10.21.276 

 (Salvin^Godman Coll.), is in the collection of the Bj'itish 

 Museum and is figured on Plate VII. 



